Alveolar fluid clearance in the developing and mature lungs is believed to be mediated, in part, by amiloride-recorded from distal lung epithelia were different from channels reported from other Na+ transporting epithelia expressing ENaC. Preliminary experiments from this laboratory show that, when grown under appropriate culture conditions, rat alveolar type II (ATII) cells express three distinct types of cation channels that are capable of transporting Na+. The highly selective and non selective cation channels appear to belong to the ENaC family of Na+ channels, but bear striking differences in their biophysical properties, and in their regulation by signal transduction pathways, including those working through protein kinases A, G, and C. It is hypothesized that lung alveolar environment, including steroids and oxygen, is responsible for selective expression of highly selective and non selective cation channels in ATII cells, and this is achieved through different arrangements of ENaC subunits. Further, it is proposed that signal transduction pathways mediated by protein kinases A, G, and C regulate the expression and activity of both the highly selective, and non selective cation channels. These questions will be addressed by combining the use of patch clamp techniques and molecular biological approaches to characterize Na+ permeant channels in ATII cells. There are three specific aims to this proposal. The first aim is to contrast the biophysical properties of Na+ channels in ATII ells at the single channel level, and to establish their relationship to the cloned ENaC subunits. The second aim is to investigate conditions, including exposure to steroid hormones and differing oxygen tension, that lead to differences in the density of non selective channels and highly selective channels in ATII cells. The third aim is to contrast the regulation of highly selective and non-selective cation channels in ATII cells by phosphorylation and dephosphorylation. These experiments will improve our understanding of how lungs modulate alveolar fluid absorption, and help it devising therapeutic strategies to improve the process.